Tetrabromobisphenol-A is one of the most widely used brominated flame retardants in the world. It is used extensively to provide flame retardancy for styrenic thermoplastics and for some thermoset resins.
Processes used for producing tetrabromobisphenol-A generally fall into three categories. The first category includes processes in which substantial amounts of methyl bromide are co-produced along with the tetrabromobisphenol-A. Generally, up to 40-50 pounds of methyl bromide can be expected per 100 pounds of tetrabromobisphenol-A produced. In most cases, the processes within this first category feature reacting bisphenol-A and bromine in methanol. The ring-bromination of the bisphenol-A is a substitution reaction which generates one mole of HBr per ring-bromination site. Thus, for the production of tetrabromobisphenol-A, four moles of HBr are generated per mole of tetrabromobisphenol-A produced. The HBr in turn reacts with the methanol solvent to produce the methyl bromide co-product. After the bisphenol-A and bromine feed are finished, the reactor contents are cooked for one to two hours to complete the reaction. At the end of the reaction, water is added to the reactor contents to precipitate out the desired tetrabromobisphenol-A product.
The second category of processes features the production of tetrabromobisphenol-A without the co-production of substantial amounts of methyl bromide and without the use of oxidants to convert the HBr to Br.sub.2. See for example U.S. Pat. Nos. 4,990,321; 5,008,469; 5,059,726; and 5,138,10. Generally, these processes brominate the bisphenol-A at a low temperature, e.g., 0 to 20.degree. C., in the presence of a methanol solvent and a specified amount of water. The water and low temperature attenuate the production of methyl bromide by slowing the reaction between methanol and HBr. The amount of water used, however, is not so large as to cause precipitation of the tetrabromobisphenol-A from the reaction mass during the bromination reaction. Additional water for that purpose is added at the end of the reaction. This type of process typically uses a fairly long aging or cook period after the reactants have all been fed, and requires additional process time for the final precipitation of tetrabromobisphenol-A via the last water addition.
In the third category are those processes which feature the bromination of bisphenol-A with bromine in the presence of a solvent and, optionally, an oxidant, e.g., H.sub.2 O.sub.2, Cl.sub.2, etc. See for example U.S. Pat. No. 3,929,907; U.S. Pat. No. 4,180,684; U.S. Pat. No. 5,068,463 and Japanese 77/034620 B4 77/09/05. The solvent is generally a water-immiscible halogenated organic compound. Water is used in the reaction mass to provide a two-phase system. As the bisphenol-A is brominated, the tetrabromobisphenol-A is formed in the solvent. The co-produced HBr is present in the water. When used, the oxidant oxidizes the HBr to Br.sub.2, which in turn is then available to brominate more bisphenol-A and its under-brominated species. By oxidizing the HBr to Br.sub.2, only about two moles of Br.sub.2 feed are needed per mole of bisphenol-A fed to the reactor. To recover the tetrabromobisphenol-A from the solvent, the solution is cooled until tetrabromobisphenol-A precipitation occurs. The cooling of the solution to recover tetrabromobisphenol-A entails additional expense and process time.
Highly effective process technology for producing tetrabromobisphenol-A is described in commonly-owned U.S. Pat. Nos. 5,527,971, 5,723,690, and 5,847,232, and in commonly-owned co-pending U.S. patent application Ser. Nos. 08/945,158, filed Apr. 18, 1996, 09/096,332, filed Jun. 11, 1998, and 09/149,225, filed Sep. 8, 1998. One of the factors involved in achieving the highly desirable results made possible by these commonly-owned processes is relatively close control of the amount of excess unreacted bromine in the liquid phase of the reaction mass during the bromination. In most cases this amount is maintained in the range of about 50 to about 10,000 ppm of unreacted bromine in the liquid phase of the reaction mass. Failure to keep the bromine level in the liquid phase of the reaction mass below this level especially when using a liquid bromine feed can have adverse repercussions, especially on the color characteristics of the tetrabromobisphenol-A product. In commonly-owned copending U.S. application Ser. Nos. 08/945,158 and 09/096,332, of which the present application is a continuation-in-part, a process is described wherein from about 50 to 20,000 ppm of unreacted bromine can be maintained in the liquid phase of the reaction mass by employing, inter alia, a feed stream of gaseous bromine to the reaction mass, if the feed stream has a Reynold's Number .gtoreq.50,000. In this way a product having improved color, e.g., an APHA color of less than about 50, and lower ionic content can be produced.
Since the commonly-owned processes referred to above involve formation of tetrabromobisphenol-A precipitate as the bromination is proceeding, such precipitate formation can render colorimetric methods for closely assessing bromine concentration in the liquid phase of the reaction mass somewhat problematical. And while feed of vaporous bromine in the manner described in application Ser. Nos. 08/945,158 and 09/096,332, referred to hereinabove, permits greater latitude in the amount of excess bromine that can be tolerated in liquid phase of the reaction mass, it remains necessary to observe and maintain careful control over the character (e.g., Reynold's number) and quantities of the vaporous bromine being fed to the reaction mass in order to ensure production of product of acceptable color characteristics.
It would thus be of considerable advantage if a way could be found of producing tetrabromobisphenol-A of desirable minimal color with process technology that is even more tolerant of the amount of unreacted bromine in the liquid phase of the reaction mixture. And it would be particularly desirable if this objective could be achieved without sacrifice of other advantageous features of the commonly-owned technology, such as forming during the bromination precipitated tetrabromobisphenol-A that is highly pure, readily recoverable, and formed in high yield based on the bisphenol-A fed to the reaction. This invention is deemed to make possible the achievement of each of the foregoing objectives.